1. Field of the Invention
The present invention relates, in general, to a fitter circuit and, more particularly, to a filter circuit having a transconductance amplifier and a detection circuit having the filter circuit.
2. Description of the Prior Art
The conventional filter circuit is disclosed in “An Accurate Center Frequency Tuning Scheme for 450-khz CMOS Gm-C Bandpass Fifters”, Hiroshi Yamazaki et al., IEEE Journal of Solid-State Circuits, vol. 34, No. 12, December 1999.
The conventional circuit having the conventional filter circuit will be described in FIG. 5. The conventional circuit comprises with a current outputting circuit 510, a current-voltage transferring circuit 520 connecting to the current outputting circuit 510 and a filter circuit 530 connecting to the current-voltage transferring circuit 520. The filter circuit 530 is a biquad bandpass filter. The filter circuit 530 has an input terminal 531, the transconductance amplifiers 532–535, the capacitors 536, 537 and an output terminal 538. Each transconductance amplifier has a voltage input terminal, a current input terminal and a current output terminal. The input terminal 531 is supplied with an input signal Vin of which type is voltage. The voltage input terminal of the transconductance amplifier 532 is connected to the input terminal 531. The current input terminal of the transconductance amplifier 532 is connected to a ground node which is supplied with the ground voltage. The current output terminal of the transconductance amplifier 532 is connected to the current input terminal of the transconductance amplifier 533, the current input terminal and the voltage input terminal of the transconductance amplifier 534, one terminal of the capacitor 536, the voltage input terminal of the transconductance amplifier 535 and the output terminal 538. The current output terminal of the transconductance amplifier 533 is connected to the ground node. The voltage input terminal of the transconductance amplifier 533 is connected to the current output terminal of the transconductance amplifier 535 and one terminal of the capacitor 537. The current output terminal of the transconductance amplifier 534, the other terminal of the capacitor 536, the current input terminal of the transconductance amplifier 535 and the other terminal of the capacitor 537 is connected to the ground node. The output terminal 538 outputs an output signal Vout.
Each transconductance value (the coefficient of voltage-current transfer) of the transconductance amplifiers 532–535 is gm. The capacitors 536, 537 have a capacitance value of C1 and C2, respectively. A transfer equation T(S) of the filter circuit 530 shown in FIG. 5 is:
                              T          ⁡                      (            s            )                          =                              Vout            Vin                    =                                    s                              gm                ·                C2                                                                    s                2                            +                              s                                  gm                  ·                  C2                                            +                              1                                                      gm                    2                                    ·                  C1                  ·                  C2                                                                                        (        1        )            
A transfer equation T(S) of a typical quadratic bandpass filter is:
                              T          ⁡                      (            s            )                          =                                                            ω                0                            Q                        ⁢            s                                              s              2                        +                                                            ω                  0                                Q                            ⁢              s                        +                          ω              0              2                                                          (        2        )            
In comparison between equations [1] and [1], the filter circuit 530 operates as the bandpass filter. In this example, tile cutoff frequency ω0 an. the quality factor Q are:
                              ω          0                =                  1                      gm            ·                                          C1                ·                C2                                                                        (        3        )                                Q        =                              C2            C1                                              (        4        )            
The filter circuit 530 covers scattering value of the element by adjusting the transconductance value of the transconductance amplifier, so the filter circuit 530 achieves high precision of the filter characteristics.
However, the conventional circuit having the conventional filter circuit has the current-voltage transferring circuit between the current outputting circuit and the filter circuit. The input terminal Vin of the filter circuit 530 does not input current but voltage. Therefore, the circuit scale of the conventional circuit having the filter circuit becomes large and the conventional circuit requires a measurable amount of power.
In addition, be transconductance, differing from the passive element such as inductor or resistor et al., have to use within the range that the amplitude of the input signal Vin does not exceed the input dynamic range. The filter circuit has to control the amplitude of the input signal Vin. The input signal Vin includes the main signal component and the frequency component of the passing band which is the same as the main signal component. Therefore, the efficiency of the filter circuit is inefficient.